How well do you know your characters?
It's a simple question, and a complicated one.
When you're writing a story, it's sometimes the case that secondary and tertiary characters are really on there for the interactions that they have with your main character(s). Your main character is there for that particular arc of story that you're telling.
So, do you think about them otherwise?
Do you know what their comfort food is? Their fear when they woke up in the middle of the night at age 3? What's in their fridge? Do they have a fridge?
Most recently, while playing Mage this past winter, my fiance had each of the player's fill out a questionnaire as our characters. It was a stroke of brilliance, really. It was fun to do, and really put you in that person's head. The things that happen "off stage" are still important, be it in a book or in a game.
All these little things that we do, they make us who we are. Our characters would do them too. If we let them.